Labeling of plastics, ceramics, glasses and metals by irradiation has long been known in the art and a number of methods exist for achieving these labels or marks. Marks can be made on the surface of a part by physically etching away the surface, by applying additional material to the surface, or by inducing a physical or chemical change at or near the surface. When irradiation of a surface is achieved with a laser, for example, all of the aforementioned techniques are possible.
Many examples exist of the use of lasers to achieve color marks on plastic or ceramic parts, yet there are few examples of methods for achieving colored marks on metal parts and in particular, chromium plated or chromed parts. With regard to plastic parts, methods for discoloring or decoloring a surface are known in the art. This may be achieved by creating a composite material combining a thermoplastic polymer, a mineral black pigment and a coloring agent. When the material is irradiated, the carbon black absorbs the radiation and vaporizes leaving the color of the colorant material to show. Additional examples exist wherein the coloring agent is formulated to produce more than one color when irradiated with a plurality of different laser sources. For example, U.S. application 2008/0139707 A1 teaches a method for producing a multi-color laser marking on a molded article. While it is possible to create composite materials comprising metals, the same techniques have yet to be applied for the production of colored marks on metal parts or surfaces on a metal or non-metal (e.g. plastic) part.
U.S. Pat. No. 6,313,436 discloses a method for laser-marking the surface of a material by applying a coating to the surface and then irradiating the coating with a laser. This method is an example of an additive process involving the use of a laser to adhere additional material to a surface. Therefore, the method may produce a color marking on a surface by including the colorant in the added coating material. Moreover, the method is less dependent on the characteristics of the part. However, one drawback is that this approach does not directly alter the appearance of the original material itself.
In direct metal marking material is removed in the micron range. Examples of metals that can be labeled include chromium, aluminum, steel, iron, titanium and tin. Composite materials such as part with a chromium-nickel layer may also be labeled with a laser wherein a layer of chromium is removed in the micron range leaving the nickel layer undamaged so that corrosion protection of the component is retained. Currently there are a number of methods for achieving black or gray marks through the use of irradiation of a metal surface. However, there are few if any examples of methods for generating color marks.
A recent study with a Titanium:sapphire laser system demonstrated the ability to alter the appearance of an aluminum surface by applying femtosecond laser pulses (Vorobyev, A. Y. and C. Guo (2008). “Colorizing Metals with Femtosecond Laser Pulses.” Applied Physics Letters 92(4)).The method resulted in gold, black or grey color marking on the aluminum. Platinum and palladium surface were also studied. While this example is a unique method for color marking a surface, the method (i) is not known to work for all metal surfaces, (ii) requires a specialized and expensive laser compared with other marking systems, and (iii) is time intensive with linear marking speeds on the order of 1 mm per second.
What is needed and is not disclosed in the art is a method of making commercially desirable colored marks on metal surface without the use of pigments, dyes or other additives materials. Preferably, the process would also be able to allow for direct conversion of metal surfaces. In addition the process should be able to make the marks as quickly as possible to allow for commercial-scale production capacity.